It found ground staff incorrectly assigned a group of more than 80 schoolchildren the adult weight of 87 kilograms, instead of the 32 kilograms usually assigned to a child aged 2 - 11.

The group was seated at the back of the Boeing 737.

The pilot found the plane was "heavy-nosed" during take off, and was forced to exceed the safe take off speed by about 25 knots.

"To rotate the aircraft and lift off from the runway, the captain found that significant back pressure was required," the report found.

"Conscious of the potential threat of striking the aircraft tail on the runway if too much back pressure was applied to the controls, the captain maintained steady back pressure to ease the aircraft into the air.

"A post-flight review determined that the final load sheet overstated the aircraft take-off weight by about 3.5 to 5 tonnes."

The crew did not experience any more problems during the flight.

The report stated Qantas has tightened its check-in procedures, to ensure correct passenger weights are assigned and the aircraft is properly balanced.